In accordance with a preferred aspect of the present invention, the upper furnace arch, i.e., the boiler nose, is employed as a plenum from which overfire air is injected into the combustion gases. With this configuration, the overfire air need penetrate only a short distance into the combustion gases to provide optimum mixing performance without the need for higher pressure boost air fans or higher pressure overfire air. Particularly, the boiler nose itself may serve as a plenum in which overfire air is received, preferably through openings in one or both of the side walls for flow through ports in the boiler nose and consequent injection into the combustion gases. Preferably, however, the overfire air is supplied to ducts extending from one or both of the side walls of the furnace into the boiler nose. A plurality of port ducts communicate between the laterally extending duct(s) in the boiler nose and ports formed along the one or more inclined surfaces of the boiler nose for injection into the combustion gases. That is, the boiler nose is generally comprised of a vertically upwardly inclined lower surface directed toward the restriction in the flue gas passage formed by the nose and the opposite boiler wall and an upper inclined surface directed away from the restriction in the flue gas passage. The overfire air injection ports may be provided in the lower or upper or both inclined surfaces of the boiler nose.
In a further embodiment, the overfire air may be supplied to the boiler nose in a pair of discrete ducts respectively extending into the boiler nose from opposite side walls of the furnace. Each of the laterally extending ducts has a plurality of port ducts communicating with the ports in the inclined wall of the boiler nose. It will also be appreciated that two or more ducts may be provided in the boiler nose extending from the respective side walls of the boiler. In that configuration, the supply of overfire air can be regulated into different zones of the combustion gases. In these various embodiments, it will be appreciated that the overfire air is supplied from injection ports in the boiler nose without the need for higher pressure boost fans or any reconfiguration of the rear wall of the furnace serving as a common wall between the furnace and the convection backpass. These embodiments also afford injection of the overfire air directly into the portion of the stratified combustion gas flow which is skewed to the rear half of the furnace.
In a preferred aspect of the present invention, there is provided a boiler comprising: a primary combustion zone having a downstream passage for flowing flue gases generated during combustion; and a boiler nose forming with walls of the boiler a restriction in the downstream flue gas passage, the boiler nose having a plurality of ports for feeding overfire air into the flue gases flowing along the downstream passage.
In a further preferred aspect of the present invention, there is provided a boiler comprising: a combustion zone; a boiler enclosure having side walls, a plurality of generally vertically extending water tubes forming at least portions of the side walls and a passage downstream of the combustion zone for flowing flue gas generated in the combustion zone; and a boiler nose formed at least in part by the water tubes and projecting toward an opposite wall of the boiler to form a restriction in the downstream flue passage, the boiler nose defining a generally longitudinally extending cavity substantially between a pair of boiler side walls, a duct extending through at least one of the pair of boiler side walls and into the cavity, and a plurality of ports spaced one from the other along the nose and in communication with the duct for injecting overfire air supplied to the duct into the downstream flue gas passage.